[SalesForce] Can Lightning Email Templates call cross object merge fields

I think you need to use visualforce email template with controller.

Controller "

    ppublic class GetChild {
    private final List<Account> accounts;


    public GetChild() {
        accounts = [SELECT Name,(SELECT CreatedBy.Name FROM Notes) FROM Account];
    }

    public List<Account> getchildNotes() {

       Return accounts;
    }
}

Create a component and name it SmithNotes and access the child values something like this

<apex:component controller="GetChild" access="global">
    <apex:variable var="count" value="{!0}"/> //declare
        <apex:dataTable value="{!childNotes}" var="s_account">
            <apex:column >
                <apex:facet name="header">Notes Title</apex:facet>
                {!s_account.Notes[count].title}
            </apex:column>
        </apex:dataTable>
        <apex:variable var="count" value="{!count + 1}"/> //increment
   </apex:component>

In your visualforce email template include the component.

<messaging:emailTemplate subject="Embedding Apex Code" recipientType="Contact" relatedToType="Opportunity">
    <messaging:htmlEmailBody>
        <p>As you requested here are the notes</p>
        <c:SmithNotes/>
        <p>Hope this helps with the {!relatedToType}.</p>
    </messaging:htmlEmailBody>
</messaging:emailTemplate>

It turned not being a syntax issue. Prior to posting this question, I should have thought about building a test formula field, which I ended up doing so.

The correct syntax would be:

{!IF(Lead.Quote_Request_Upholstery_Cleaning__c = TRUE, Lead.Quote_No_Bedrooms_HC__c, NULL)}